This invention is related to partially crosslinked olefinic thermoplastic elastomers (TPEs).
Although compounds comprising a polyolefin and a partially crosslinked rubber possess good properties as TPEs such as heat resistance, mechanical strength, flexibility, and elasticity, their flow properties are inferior to those of common plastics, and this has limited their use in large molded parts owing to problems such as "flow marks." To improve these problems, for example, Japanese Patent 54-23702 teaches the use of compounds consisting of polyolefin and two types of rubber, partially crosslinked, and Japanese Patent 56-15743 teaches a compound in which a polyolefin is blended with a polyolefin/rubber blend that has been treated with organic peroxides. Also Japanese Patent 56-15740 teaches the methods of manufacture of such compounds. On the other hand, as examples of TPEs containing styrenic rubber, Japanese Patent Application 59-6236 discloses compositions containing, for example, hydrogenated derivatives of styrene-conjugated diene copolymers, peroxide-crosslinked olefinic rubbers, uncrosslinked hydrocarbon rubbers and inorganic fillers and manufacturing processes for these, and Japanese Patent Application 60-166339 discloses partially crosslinked compositions consisting of hydrogenated derivatives of styrene-conjugated diene copolymers, rubber softening agents, peroxide decomposing type olefinic resins and conjugated diene rubbers. However, the peroxides used in the manufacture of the above partially crosslinked TPEs are highly reactive, making it difficult to control the degree of crosslinking, and in addition because chain scission occurs simultaneously, the compound physical properties become poor, the resulting low molecular weight species lead to poor paintability and partial gelation leads to a poor surface appearance. In addition, there have been problems of poor heat stability due to the free radicals remaining from the organic peroxide.
In order to resolve these problems, the inventors have blended halobutyl rubber into the rubber component, and have proposed compositions with a polyolefin added to a rubber component/polyolefin that are partially crosslinked using metal oxides (MOx) and/or metal chlorides (MCl) (Japanese Application 60-188419).
Although the abovementioned compositions possess good properties as TPEs, they are inadequate in gloss to give a high quality appearance compared to polyurethane and polyester TPEs. It is one objective of this invention to improve the gloss of molded parts without loss in the advantages of TPE polyolefin compounds, i.e. elasticity, and flow and mechanical properties suitable for large molded parts.